1. Field of the Invention
This invention relates to adjusting the temperature and pressure of steam for use in a steam turbine, and more particularly, to means for supplying steam having varying degrees of superheat from steam sources that are dry and saturated, superheated, and a mixture thereof.
2. Description of the Prior Art
In central station power generation facilities large, relatively massive steam turbines are generally used to drive alternating current generators. Such steam turbines generally operate with a maximum temperature of 800.degree.-1000.degree. F. and 1000-2500 pounds per square inch throttle pressure. During steam turbine start-up, the temperature of the components therein must be gradually warmed to avoid inducing abnormally large thermal stresses.
Limits exist on the rate of temperature increase of the steam and the allowable temperature differences between the steam and the turbine components. Due to such limits, the temperature and pressure of steam flowing through a steam turbine during start-up are usually gradually increased from reduced values to operational values at the allowable rates. Steam temperatures from fossil fuel fired boilers are somewhat difficult to control at relatively low turbine loads and as a consequence, during turbine start-up, the boiler is sometimes the limiting factor for determining the precision in the rate of change of steam temperature and pressure supplied to the turbine. Temperature and pressure increases in boilers during turbine start-up are sometimes greater than allowable for use in the turbine as judged from reliability considerations. Precise control of steam temperature and pressure increases in a boiler has been attempted by regulating fossil fuel firing rate with limited success but, in general, such regulation has proven to be difficult.
Liquid metal fast breeder reactor (LMFBR) power plants are believed (by cost projections) to have higher capital costs and lower operating costs than conventional, fossil fuel power plants. For a utility to avail itself of such lower operating costs, on line availability and system component reliability in LMFBR systems must be maximized to ensure continuous operation. System and component reliability may be adversely affected by imprecise control of the temperature and pressure of the steam which was transmitted to the turbine throttle. For LMFBR plants, steam, during turbine start-up, is typically dry and saturated from the steam generator's steam drum or slightly superheated if taken from the steam generator's superheater. The steam pressure in LMFBR systems is typically approximately 5% higher than the main turbine design throttle pressure. Should such steam be throttled and sent to the main turbine, as proposed by some designers, at approximately 1000 psi (a reasonable start-up pressure), the steam would have between 2% and 12.5% moisture at the main turbine throttle. The moisture percentage will increase further because of throttling in the turbine throttle and/or control valves. Such moisture level can cause serious erosion and other operational problems including severe turbine chilling.